Dump Load

[2windy]

I built a dump load out of an old electric dryer element. It has 6 coils in  it, 2.0 ohms each. I parralleled them all together and ended up with .32 ohms. At 13.5 volts its 40 amps, exactly what I needed for my C-40 controller. Thanks to Flux for setting me straight on  this matter of dumping power. It works perfect so far except for one thing, it buzzes when its on. Was wondering if this is normal for something like this?  I was watching it today dumping power cause my batteries were charged. I have an amp meter hooked up to it to watch how much its dumping. Also I hooked up my digital voltmeter to see what the voltage would be. At 30 amps it was bettween 6 and 7 volts. As the amps went down so did the voltage. I have a 12 volt system. Is my controller working proper, and is this buzzing normal?

[Flux]

The buzzing is from the pwm switching of the controller and is normal.

It sounds as though everything is working ok. Measuring volts and amps into the dump load is interesting but something that is not normally done.

As long as battery volts are staying within their normal limits everything seems fine.

Flux

[SamoaPower]

Yes, magnetostriction of the load element makes the PWM audible. Same as a humming transformer.

The voltage will measure low on the usual meter because of the PWM waveform. If you were to use a scope, you would see peaks going up to the battery voltage.

You're in business.

[altosack]

A bit off-topic, but maybe an electronics expert can help me with this.

My MorningStar TS-45 also buzzes like this, but at a higher frequency that most would term a "whine" varying from about 2-7 kHz (by my ear, but my musicianship has fallen by the wayside). Although it's not that loud, I find it annoying and because of that, when I build by own, I want to use a frequency in the range of 30-35 kHz (this coincides with the frequency I will be using for the buck converter).

My question is: does anyone know why the manufacturers choose a frequency this low, and why it would need to vary in frequency ?  I don't think it's cost or efficiency; I can find MOSFETs for less than $2 US that switch fast enough that they're dissipating less than 2% at 100 kHz.

Any ideas ?

Dave

[ghurd]

My meter says the SG and SS versions run around 360Hz.

If they made noise, tightening the mounting screws fixed it, but you may have noise in the dump load.

G-

[Flux]

These PWM controllers just switch the load on and off. If the frequency changes then it is most likely working with constant pulse width.

If you raise the frequency,the load may become more critical as coils of wire, including heaters become significantly inductive above the audio frequencies and this simple on off technique is only suitable for resistive load.

You could add an inductor and make it a true PWM buck converter to drive the dump load. This adds a cost and a bit of complexity that is hardly justified to produce waste heat. Also I think there is some evidence to support the view that batteries like a pulsating charge, but this may not be an issue against a true PWM load with a freewheel diode to run with constant current.

Flux

[ghurd]

I don't believe the frequency changes. The duty cycle changes, 0 to 100%.

"My old meter" reads all the Morningstar products I ever tested at 359Hz, but I never used a TS.

G-

[Ungrounded Lightning Rod]

You could add an inductor and make it a true PWM buck converter to drive the dump load.

Inductor in series with the hot wire.  Plus a back-biased "freewheeling" diode from ground to the inductor/controller junction to provide current during the "off" part of the cycle.

But at audio frequencies (even high ones) you'll need a rather large inducance, and with 40 amps that means a big core to avoid saturation.  (And then the inductor will probably buzz, too.)

Be sure you get the diode in the right way or you'll kill something - probably the controller.

Or just put some (heatproof!) sound insulation between you and the dump load - without restricting airflow significantly, of course.

[2windy]

Battery volts are staying right were I set them in the controller. As for the meters hooked to the load, I was curious to see how many amps and volts was getting dumped. I was just wondering about the voltage why it was so low and the amps so high. But then you have explained that to me before, like water running over the dam. I can live with the buzzing as long as it's normal for something like this.

[cdog]

Would a hot water heater type dump load likely be somewhat less noisy??

[GeorgeB71]

I was hoping to use the excess power from the "dump load" circuit to heat water.  Would it be possible to get a regular 120v water heater and hook the wires from the heating element to the "dump load" circuit?  I have a 12 volt system.  Would this be enough to heat up the heating element?  If not, any ideas on how to make a "12 volt" heating element to use in the heater?  Thanks...